Wire bonding method and semiconductor device

ABSTRACT

After forming a pressure-bonded ball and a ball neck by bonding an initial ball to a pad, a capillary is moved upward, away from a lead, and then downward, thereby the ball neck is trodden on by a face portion that is on the lead side of the capillary. Subsequently, the capillary is moved upward and then toward the lead until the face portion of the capillary is positioned above the ball neck, thereby a wire is folded back toward the lead. Then, the capillary is moved downward such that a side of the wire is pressed by the capillary against the ball neck that has been trodden on. After the capillary is moved obliquely upward toward the lead and then looped toward the lead, the wire is pressure-bonded to the lead.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a wire-bonding method of connecting a first bonding point with a second bonding point by a wire, and to a semiconductor device having a wire-loop shape formed by connecting a first bonding point with a second bonding point by a wire.

2. Description of the Related Art

For assembling a semiconductor device, wire bonding for connecting a pad of a semiconductor chip mounted on a lead frame with a lead of the lead frame by a thin metal wire is used. A wire-bonding apparatus is used in the wire bonding, in which an initial ball is first formed at a tip end of the wire, and the initial ball is pressure-bonded to the pad of the semiconductor chip using the capillary, thereby forming a pressure-bonded ball. In this method, after moving the capillary upward to make a reverse motion in a direction away from a second bonding point, the capillary is further moved upward to a predetermined height, and then moved toward the second bonding point, thereby the wire is connected to the second bonding point (see FIG. 4 to FIG. 6 of Japanese Unexamined Patent Application Publication No. 2004-172477 (hereinafter referred to as “Patent Document 1”), for example).

The wire is bonded by moving the capillary in this manner, and, in many cases, a shape of a wire loop is formed either in a triangular shape including a wire neck that extends upward from the pressure-bonded ball that has been pressure-bonded onto the pad of the semiconductor chip and a sloped portion that has been bent toward the second bonding point at the wire neck, or in a trapezoidal shape including a flat portion that extends substantially horizontally in a direction of the second bonding point from the wire neck and a sloped portion that extends from the flat portion toward the second bonding point. This is because, when a portion close to the pressure-bonded ball is moved horizontally in the direction of the second bonding point with respect to the capillary, the neck portion is often damaged due to a friction between the capillary and the thin metal wire that is produced while moving.

However, the height of the wire loop in this wire-loop shape is high because the wire neck that rises from the pressure-bonded ball is included, and this poses a problem that the height or thickness of the semiconductor device assembled by wire bonding as a whole cannot be made small.

Therefore, a method has been proposed, making a reverse motion such that a capillary is moved slightly upward and away from a second bonding point after bonding to a first bonding point, making a forward motion such that the capillary is further moved slightly upward and in a direction toward the second bonding point, then moving the capillary downward to press a wire neck portion against a pressure-bonded ball and fold the wire neck portion up on the pressure-bonded, making the wire to extend either in a horizontal direction or in a direction slightly sloped upward from the horizontal direction, moving the capillary upward while feeding the wire from a tip end of the capillary, and then moving the capillary toward the second bonding point, thereby connecting the wire to the second bonding point (see FIG. 1 to FIG. 3 of Patent Document 1, or FIG. 1 to FIG. 3 of Japanese Unexamined Patent Application Publication No. H09-51011 (hereinafter referred to as “Patent Document 2”), for example).

According to the conventional bonding method described in the Patent Document 1 or 2, as a wire is folded up onto a pressure-bonded ball and then pressed to form a head portion, the head portion cannot be made particularly low. As a result, the conventional bonding method occasionally fails to satisfy the demand for decreasing the height of the wire loop as a whole.

BRIEF SUMMARY OF THE INVENTION

In view of the above problem, an object of the present invention is to reduce the height of the wire loop when connecting a first bonding point with a second bonding point.

A wire-bonding method according to the present invention is a wire-bonding method of connecting between a first bonding point and a second bonding point by a wire, and the method includes: a first bonding step of bonding an initial ball formed at a tip end of the wire onto the first bonding point using a capillary, thereby forming a pressure-bonded ball and a ball neck; a ball neck treading step, after the first bonding step, of moving the capillary upward, away from the second bonding point, and then downward, thereby treading on the ball neck facing toward the second bonding point with a face portion on the second bonding point side of the capillary; a pressing step, after the ball neck treading step, of moving the capillary upward and then toward the second bonding point until the face portion away from the second bonding point of the capillary is positioned above the ball neck such that the wire is folded back toward the second bonding point, and subsequently moving the capillary downward, thereby pressing a side surface of the wire that has been folded back on the ball neck that has been trodden on by the face portion away from the second bonding point of the capillary; an oblique-upward rising step, after the pressing step, of moving the capillary obliquely upward toward the second bonding point; and a second bonding step, after the oblique-upward rising step, of moving the capillary upward and then toward the second bonding point, and bonding the wire onto the second bonding point by pressure-bonding.

In the wire-bonding method according to the present invention, it is also preferable that, in the ball neck treading step, a continuous movement be repeated for a plurality of times after the first bonding step, the continuous movement including moving the capillary upward, away from the second bonding point, and then downward, thereby treading on the ball neck facing toward the second bonding point with the face portion on the second bonding point side of the capillary.

Moreover, it is also preferable that the wire-bonding method according to the present invention further include a kink forming step, between the oblique-upward rising step and the second bonding step, of moving the capillary upward and then carrying out a reverse motion at least once in which the capillary is moved away from the second bonding point, thereby forming a kink in the wire.

A semiconductor device according to the present invention is a semiconductor device having a wire-loop shape that connects between a first bonding point and a second bonding point by a wire, and the semiconductor device is produced by a process including: a first bonding step of bonding an initial ball formed at a tip end of the wire onto the first bonding point using a capillary, thereby forming a pressure-bonded ball and a ball neck; a ball neck treading step, after the first bonding step, of moving the capillary upward, away from the second bonding point, and then downward, thereby treading on the ball neck facing toward the second bonding point with a face portion on the second bonding point side of the capillary; a pressing step, after the ball neck treading step, of moving the capillary upward and then toward the second bonding point until the face portion away from the second bonding point of the capillary is positioned above the ball neck such that the wire is folded back toward the second bonding point, and subsequently moving the capillary downward, thereby pressing a side surface of the wire that has been folded back on the ball neck that has been trodden on by the face portion away from the second bonding point of the capillary; an oblique-upward rising step, after the pressing step, of moving the capillary obliquely upward toward the second bonding point; and a second bonding step, after the oblique-upward rising step, of moving the capillary upward and then toward the second bonding point, and bonding the wire onto the second bonding point by pressure-bonding.

Also, in the semiconductor device according to the present invention, it is also preferable that, in the ball neck treading step, a continuous movement be repeated for a plurality of times after the first bonding step, the continuous movement including moving the capillary upward, away from the second bonding point, and then downward, thereby treading on the ball neck facing toward the second bonding point with the face portion on the second bonding point side of the capillary.

The present invention provides an advantageous effect of reducing height of a wire loop in connecting a first bonding point with a second bonding point.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1 (a) through 1 (f) are an illustrative diagram showing a treading step and a pressing step in a bonding method of an exemplary embodiment according to the present invention;

FIGS. 2 (a) through 2 (e) are an illustrative diagram showing a kink forming step and a second bonding step in the bonding method of an exemplary embodiment according to the present invention, and a wire loop of an exemplary embodiment according to the present invention;

FIG. 3 is an illustrative diagram showing a movement of a tip end of a capillary in the bonding method of an exemplary embodiment according to the present invention;

FIGS. 4 (a) through 4 (f) are an illustrative diagram showing a treading step in the bonding method of a different exemplary embodiment according to the present invention; and

FIG. 5 is an illustrative diagram showing a movement of a tip end of a capillary in the bonding method of a different exemplary embodiment according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following describes an exemplary embodiment according to the present invention with reference to FIGS. 1 to FIG. 3. As shown in FIG. 1 and FIGS. 2, this exemplary embodiment relates to a wire-bonding method of connecting a pad 13 of a semiconductor chip 11 mounted on a lead frame 12 with a lead 17 formed on the lead frame 12, and to a semiconductor device having a wire loop 20 formed by this wire-bonding method. While a semiconductor device is provided with wire loops that connect a plurality of pads 13 with leads 17, respectively, the connection between a single pad 13 and a single lead 17 is described in the following. The pad 13 of the semiconductor chip 11 corresponds onto a first bonding point, and the lead 17 of the lead frame 12 corresponds onto a second bonding point. Although the lead 17 is not shown in FIG. 1, the lead 17 is at the right side in the drawing. Also, FIG. 3 is a diagrammatic view showing a movement of the tip end of a capillary 41.

First, a first bonding step is carried out, in which an initial ball that is not shown in the Figures and formed at an end portion of a wire 21 is pressed and bonded onto the pad 13 by the capillary 41 while applying ultrasonic vibration, thereby a pressure-bonded ball 23 and a ball neck 25 are formed on the pad 13. The pressure-bonded ball 23 is formed by flattening out the spherical initial ball with a face portion 43 of the capillary 41 into a disk-shape, and the ball neck 25 is formed by a portion of the initial ball intruding between an inner chamfer 45 and a straight hall 47 of the capillary 41. The ball neck 25 extends toward the wire 21 from the pressure-bonded ball 23, and has a circular cylindrical shape whose diameter is substantially as large as that of the straight hole 47 of the capillary 41, whose diameter is greater than that of the wire 21. When pressing the capillary 41 against the pad 13, the tip end of the capillary 41 is positioned at a point a above the pressure-bonded ball 23 shown in FIG. 3.

After the first bonding step, a treading step as shown from FIG. 1 (a) to FIG. 1 (c) is carried out. In the treading step, as shown by FIG. 1 (a), the tip end of the capillary 41 is moved to a point b from the point a shown in FIG. 3, by moving the capillary 41 upward while feeding the wire 21. Then, as shown by FIG. 1 (b), the capillary 41 is moved away from the lead 17 until the face portion 43 which is on the lead 17 side of the capillary 41 is positioned above the ball neck 25, thereby moving the tip end of the capillary 41 to a point c from the point b shown in FIG. 3. At this time, the wire 21 is sloped from the ball neck 25 toward a direction opposite from the lead 17. Then, the tip end of the capillary 41 is moved down to a point d from the point c shown in FIG. 3, and a portion of the ball neck 25 facing toward the lead 17 is trodden on by the face portion 43 which is on the lead 17 side of the capillary 41 as shown by FIG. 1 (c). By the treading, the cylindrical ball neck 25 is formed into a disk-shaped trodden portion 25 a whose diameter is slightly smaller than that of the pressure-bonded ball 23. Being fattened by the face portion 43 of the capillary 41, an upper surface of the trodden portion 25 a on the lead 17 side is made into a flat plane that fits the shape of the face portion 43. Moreover, the wire 21 is bent toward a portion of the trodden portion 25 a facing away from the lead 17, and extends upright in a direction perpendicular to the pad 13 along an inner surface of the straight hole 47 facing away from the lead 17 of the capillary 41.

Next, a pressing step as shown from FIG. 1 (d) to FIG. 1 (f) is carried out. As shown by FIG. 1 (d), the tip end of the capillary 41 is moved upward to a point e from the point d shown in FIG. 3 while feeding the wire 21 from the tip end of the capillary 41. With this, the wire 21 is fed out linearly along the straight hole 47 of the capillary 41. Then, the tip end of the capillary 41 is moved toward the lead 17, to a point if from the point e shown in FIG. 3. With this, as shown by FIG. 1 (e), the wire 21 is pushed toward the lead 17 by the inner chamfer 45 of the capillary 41 and bent at a bent portion 25 b that continues from the trodden portion 25 a. Then, the capillary 41 is moved toward the lead 17 until the face portion 43 away from the lead 17 of the capillary 41 is positioned above the pressure-bonded ball 23. Then, the tip end of the capillary 41 is moved down to a point g. from the point f shown in FIG. 3. As a result, as shown by FIG. 1 (f), by this downward movement of the capillary 41, a side surface of the wire 21 is pressed against the upper surface of the trodden portion 25 a that has been formed by treading on the ball neck 25. With this pressing of the wire 21, the bent portion 25 b of the wire 21 is folded back to the trodden portion 25 a, and thus a folded-up portion 26 a is formed. A pressed portion 26 of the wire 21 facing toward the pad 13 is pressed against the upper surface of the trodden portion 25 a by the pressing, and an upper surface of the pressed portion 26 is formed into a flat plane by the face portion 43 of the capillary 41. Upon completion of the pressing step, the capillary 41 is positioned closer to the lead 17 with respect to a bonding center line 28 of the pad 13.

When pressing the wire 21 against the trodden portion 25 a with the face portion 43 of the capillary 41, the capillary 41 can be moved reciprocatingly in a direction toward and away from the lead 17 at the same time with the pressing so that the wire 21 is fitted with the trodden portion 25 a. Alternatively, the pressing of the wire 21 can be carried out while vibrating the tip end of the capillary 41 by ultrasonic vibration.

As shown by FIGS. 2 (a), after the pressing step, an oblique-upward rising step is carried out in which the capillary 41 is moved obliquely upward toward the lead 17, to a point h from the point g shown in FIG. 3 while moving the capillary 41 upward with the wire 21 being fed out from the tip end of the capillary 41. With this oblique-upward rising step, a kinked portion that projects toward the lead 17 is formed in the wire 21.

As shown by FIG. 2 (b), after the oblique-upward rising step, a kink forming step is carried out. In the kink forming step, after the tip end of the capillary 41 is moved upward to a point i from the point h shown in FIG. 3 while feeding out the wire 21 from the tip end of the capillary 41, a reverse motion toward the direction away from the lead 17 is made and the tip end of the capillary 41 is moved to a point j from the point i shown in FIG. 3. With this movement, the capillary 41 that has been positioned closer to the lead 17 with respect to the bonding center line 28 of the pad 13 upon completion of the previous pressing step is positioned away from the lead 17. With this reverse motion, the wire 21 that has risen from a portion of the pad 13 facing toward the lead 17 is formed into a sloped shape curving toward the direction away from the lead 17. In contrast, as the wire 21 within the capillary 41 is held substantially perpendicular to a surface of the pad 13, a kink 34 that projects toward the direction away from the lead 17 is formed in the wire 21 near the tip end of the capillary 41 when the reverse motion is completed.

As shown from FIG. 2 (c) to FIG. 2 (e), a second bonding step is carried out subsequently to the reverse motion. As the tip end of the capillary 41 is moved upward to a point k from the point j as shown in FIG. 3 while feeding out the wire 21, the wire 21 is fed out continuously after the kink 34 as shown by FIG. 2 (c). A portion of the wire 21 that is being fed by the upward movement of the capillary 41 is longer than a portion of the wire that has been fed during the previous reverse motion. Then, the tip end of the capillary 41 is moved to a point m shown in FIG. 3 from the point k shown in FIG. 3, and the capillary 41 is moved toward the lead 17 in a looping arch as shown by FIG. 2 (d) and FIG. 2 (e). By this looping, the kink 34 is further bent into a flexed portion 27. Moreover, the kinked portion projecting toward the lead 17 that has been formed in the wire 21 in the oblique-upward rising step is formed into a shape such that the wire 21 that extends from the pressed portion 26 toward the lead 17 projects downward by this looping. Then, the wire 21 is pressure-bonded to the lead 17 by pressing the tip end of the capillary 41 against the lead 17, thereby the bonding is completed. Upon completion of the bonding of the wire 21 to the lead 17, the wire loop 20 that connects the pad 13 serving as the first bonding point with the lead 17 serving as the second bonding point is formed. The assembly of the semiconductor device is completed when all the pads 13 of the semiconductor chip 11 are respectively connected with all the leads 17 of the lead frame 12 by the wire loops 20.

According to the wire-bonding method of this exemplary embodiment, after the height of the ball neck 25 is made low by treading on the ball neck 25 with the face portion 43 of the capillary 41 in the treading step, the wire 21 is folded back and the side surface of the wire 21 is pressed against the trodden portion 25 a which has been made lower, and then the wire 21 is looped toward the lead 17. Accordingly, as shown by FIG. 2 (e), a head height H1 of the wire loop 20 of the semiconductor device formed on the pad 13 can be made lower than the height of the loop disclosed by the conventional technique, and thus the height of the wire loop 20 rising from the semiconductor chip 11 can be made even lower. Moreover, as the side surface of the wire 21 is pressed against the substantially flat upper surface of the trodden portion 25 a in the pressing step, it is possible to prevent a height H2 of the wire loop 20 between the pressed portion 26 and the lead 17 from becoming high because the wire 21 extending from the pressed portion 26 toward the lead 17 is curved projecting upward when looping, and thus the height of the wire loop 20 rising from the semiconductor chip 11 can be made even lower. In addition, the kinked portion projecting toward the lead 17 that has been formed in the wire 21 in the oblique-upward rising step is formed into the curved shape such that the wire 21 between the pressed portion 26 and the lead 17 projects downward by the looping. With this, it is possible to prevent a case in which the wire 21 extending from the pressed portion 26 toward the lead 17 is curved projecting upward, thereby increasing the height H2 of the wire loop 20 and resulting in an increased height of the wire loop 20 as a whole, and it is also possible to evenly reduce the height of the wire loop 20 as a whole. In this exemplary embodiment, the pressed portion 26 does not stick out of the pressure-bonded ball 23 in a direction of the diameter of the pressure-bonded ball 23.

According to the exemplary embodiment described above, the flexed portion 27 is formed in the wire loop 20 by carrying out the oblique-upward rising step and the kink forming step after the pressing step. However, when there is a little difference in height between the pad 13 and the lead 17, it is possible to connect the pad 13 with the lead 17 by carrying out the second bonding step after the oblique-upward rising step without carrying out the kink forming step.

Another exemplary embodiment of the present invention is now described with reference to FIG. 4 and FIG. 5. Like components are denoted by the same numerals as those used in the exemplary embodiment described above with reference to FIGS. 1 to FIG. 3, and will not be described in detail. This exemplary embodiment relates to a wire-bonding method of wire bonding when the tip end of the capillary 41 is tapered or when using the capillary 41 having a fine tip end, and further relates to a semiconductor device having a wire loop formed by this wire-bonding method.

As shown in FIG. 4, when the tip end of the capillary 41 is fine, the area of the face portion 43 of the capillary 41 is small. Accordingly, it is not possible to sufficiently tread on the ball neck 25 with a single treading, and this often increases the height of the folded-up portion 26 a of the wire 21. Therefore, as shown in FIG. 4 and FIG. 5, a series of continuous movement of moving the capillary 41 upward, away from the lead 17, then downward, and treading on the ball neck is repeated twice after the first bonding, such that the ball neck 25 can be sufficiently trodden on.

As shown from FIG. 4 (a) to FIG. 4 (c), after the first bonding, the capillary 41 is moved upward, then moved away from the lead 17, and thereafter moved downward, and then the portion of the ball neck 25 facing toward the lead 17 is trodden on by the face portion 43 which is on the lead 17 side of the capillary 41. In this movement, the tip end of the capillary is moved from the point a, to the point b, to the point c, and then to the point d shown by FIG. 5. However, a distance of the horizontal movement from the point b to the point c is such that a side surface of the face portion 43 on the lead 17 side of the capillary 41 is positioned only slightly away from the lead 17 compared to a side surface of the pressure-bonded ball 23 facing toward the lead 17. By thus moving the capillary 41 downward to tread on the ball neck 25 after having moved the capillary 41 in the horizontal direction in this manner, it is possible to make the upper surface of the ball neck 25 on the lead 17 side suitably into the trodden portion 25 a having a flat plane by the face portion 43 which is on the lead 17 side of the capillary 41.

Then, as shown from FIG. 4 (d) to FIG. 4 (f), the capillary 41 is moved upward again, then moved horizontally to the direction away from the lead 17, and moved downward again, and then the portion of the ball neck 25 positioned away from the lead 17 with respect to the trodden portion 25 a that has been formed by the previous treading is trodden on by the face portion 43 on the lead 17 side of the capillary 41, thereby increasing the area of the flat surface of the trodden portion 25 a. In this movement, the tip end of the capillary moves from the point d, to the point b′, to the point c′, and then to the point d′ shown by FIG. 5. The distance of the horizontal movement from the point b′ to the point c′ is a necessary distance in order to increase the area of the flat surface of the trodden portion 25 a.

Subsequently, after completing the continuous movement of repeating the treading step twice, similarly to the previous exemplary embodiment, the pressed portion 26 is formed by moving the capillary 41 upward, toward the lead 17, and then downward so that the wire 21 is pressed against the trodden portion 25 a as shown from FIG. 1 (d) to FIG. 1 (f). Then, the oblique-upward rising step is carried out by moving the wire 21 obliquely upward toward the lead 17, the kink forming step is carried out, and then the wire 21 is bonded to the lead 17 by looping the capillary 41 toward the lead 17. In this exemplary embodiment, the pressed portion 26 does not stick out of the pressure-bonded ball 23 in the direction of the diameter of the pressure-bonded ball 23.

According to the exemplary embodiment described above, the same advantageous effect as that of the previously described exemplary embodiment is provided, and the low wire loop 20 can be formed without fail even with the capillary 41 whose tip end is tapered or the capillary 41 with a fine tip end. Also, according to this exemplary embodiment, the series of continuous movement including the upward movement, the horizontal movement toward the direction away from the lead 17, and the downward movement of the capillary 41, and the treading by the capillary 41 in the treading step is repeated twice. However, a number of repetition of the series of continuous movement is not limited to twice, and the series of continuous movement can be repeated as many times as desired depending on the size of the tip end of the capillary 41. 

1. A semiconductor device comprising at least one wire having a loop shape making an electrical connection with the wire between a first bonding point and a second bonding point, the semiconductor device comprising: a ball neck integrally formed with a bonded ball using an initial ball formed at the leading end of the wire, the bonded ball being bonded to the first bonding point with a capillary; a pressed portion of the wire being folded back on the ball neck so as to remain within a diametrical range of the bonded ball; a wire located between the pressed portion and the second bonding point; and a bonded portion of the second bonding point and the wire bonded to each other, wherein the ball neck comprises a flat surface formed in a manner such that, after the initial ball is bonded to the first bonding point with the capillary, the capillary is raised and shifted in a direction opposite to the second bonding point, and subsequently the capillary is contacted and lowered to step on a portion of the ball neck at the second bonding point side with a first face portion located at the second bonding point side of the capillary, wherein the pressed portion is formed in a manner such that, after the capillary is contacted and lowered to step on the portion of the ball neck at the second bonding point side until the face thereof is flattened, the capillary is raised and shifted toward the second bonding point until a second face portion located at the side opposite to the second bonding point of the capillary is positioned above the ball neck, thereby folding back the wire toward the second bonding point, and subsequently the capillary is lowered to fold back the wire on the portion of the ball neck flattened being stepped on with the capillary, thereby pressing the side face of the wire with the second face portion of the capillary located at the side opposite to the second bonding point, and wherein the wire between the pressed portion and the second bonding point comprises a downward projecting curved portion and an upward projecting flexed portion on the second bonding point side of the downward projecting curved portion, the downward projecting curved portion and upward projecting flexed portion being formed in a manner such that, after the side surface of the wire is pressed with the second face portion of the capillary located at the side opposite to the second bonding point, the capillary is obliquely moved upward toward the second bonding point and subsequently raised and moved in a reverse direction in which the capillary is moved to the side opposite to the second bonding point, the movement is made at least one time; and then the capillary is further raised and moved to the second bonding point, thereby bonding the wire to the second bonding point.
 2. The semiconductor device according to claim 1, wherein the flat face of the ball neck is formed by repeatedly carrying out a continuous movement in a manner such that, after the initial ball is bonded at the first bonding point with the capillary, the capillary is raised and subsequently shifted in the direction opposite to the second bonding point, then the capillary is lowered to contact and step on the other portion of the ball neck located at the side opposite to the portion of the ball neck located at second bonding point side with the first face portion of the capillary located at the second bonding point side until the face of the wire is flattened increasing the flat area of the face of the ball neck.
 3. The semiconductor device according to claim 1, wherein the ball neck is formed into a disk-shaped stepped portion whose diameter is smaller than that of the bonded ball.
 4. The semiconductor device according to claim 2, wherein the ball neck is formed into a disk-shaped stepped portion whose diameter is smaller than that of the bonded ball. 